This invention relates generally to a device for testing sterilization apparatus and, more particularly, to such a device for testing the level, uniformity and duration of the temperature in sterilization apparatus used in hospitals and the like.
The most fundamental responsibility of a hospital is to return patients to society in no worse condition than when they entered. Probably the greatest hindrance to fulfilling this responsibility is infections that patients acquire while in the hospital. The effectiveness of hospitals in protecting their patients (and themselves against lawsuits) is directly related to the effectiveness of their infection control program. This is essentially a health care quality assurance program.
Since most of today's hospitals do a fairly good job of supplying sterile supplies and equipment, a tigntened up sterilization quality program usually will not greatly reduce the infection rate. The primary reason for a good quality sterilization assurance program is not so much to reduce the current infection rate but is rather to reduce the chances for catastrophic outbreak of postoperative infection. Since large amounts of materials are processed and quickly used, a failure in the sterilization process has the potential for disaster. Sterilization failure during a single shift could result in hundreds of infections before the cause was found an eliminated. Therefore, it is imperative that the hospital use all reasonable quality control measures to insure against sterilization failure.
Sterilization by steam is the preferred method of sterilization in hospitals throughout the world. Alternate systems such as gas, liquids, and hot air are used only where steam sterilization cannot be used. The reasons why steam sterilization is preferred are that: (1) Its action is reliable, well understood, and predictable. (2) Steam is readily available from the hospital steam plant. (3) The equipment and its operation and maintenance are relatively inexpensive. )4) In contacting cool surfaces, the steam condenses, giving up tremendous heat and through contraction draws more steam into the cooler area which is thus quickly heated.
Steam kills microorganisms by coagulating the proteins. To do this efficiently, the steam must be saturated or in other words "almost wet" or on the border between the gaseous and vapor states. Dry steam (superheated steam) is a very inefficient sterilant, about equivalent to hot air. If the steam is superheated, it will not quickly penetrate dressing packs since the only mechanism for heating will be slow convection, resulting in cool air pockets. This is in contrast to the fast convection occurring when saturated steam condenses on cool surfaces, contracts, thus bringing in fresh steam and forcing out the residual air.
The chief technical difficulty in steam sterilization is the removal of air. Air causes difficulty because it forms pockets at the center of packs and forms insulating layers over the surfaces. In ordinary downward displacement sterilizers, the steam (being lighter than air) moves downward from the fop, assisted by successive condensation and evaporation. The air is bled off at the bottom of the sterilizer through the steam trap until hot steam contacts the trap and causes it to close. Hopefully, when the trap closes all air has been removed from the chamber. However, if packs are wrapped too tightly or are too large, if the steam trap is out of adjustment and closes too quickly, if steam goes around rather than through a dense portion of the load, or if other porblems occur, air will be left in the sterilizer and sterilization failure will occur. More modern sterilizers use vacuum extraction to assist in removal of air, thus shortening the sterilization cycle. However, experience has shown that this process can be defeated by inadequate pumping or by air leaks. Sometimes steam injection during the vacuum cycle is used but even in these type units a failure due to cool air pockets can occur.
There are many reasons why sterilization quality control is essential. First, viable microorganisms are invisible and thus sterilization failures are not immediately apparent. Only indirect means can be used to detect failures. Second, materials in hospitals are used very quickly after being sterilized. Unlike commercially produced materials and drugs, they are not quarantined until proven safe. Materials sterilized in hospitals are sometimes used while still hot from the sterilzer. Finally, unsterile items normally produce a delayed reaction. Three or four days may easily elapse before the unsterile goods cause their damage, and probably several more days would elapse before the cause of trouble was pinned down.
Sterilization failure may be due to human error (packs too large or wrapped too tightly, too many packs crowded into sterilizer, by-passing the sterilizer with several packs, etc.). It may also be due to sterilizer malfunction (plugged lint screen, clogged exhaust line, defective air release valve or steam trap, etc.). Nearly all of these troubles result in cool air pockets being trapped with packs. While vacuum sterilizers are constructed to eliminate air pockets through use of a vacuum cycle, a leaky gasket on inefficient vacuum pump can leave small amounts of air which pocket at the center (or most dense portion) of packs.
There are various mechanical and biological devices and methods presently available for testing sterilization apparatus. The most widely used of these include.
1. Bacteriological sampling of the materials after they have been processed through the sterilizer. This is the only way to directly prove that a particular item is sterile. However, in performing the test, the sample is usually contaminated, and large numbers of tests are required to insure that an adequate statistical sample has been taken. Such sampling is not generally considered practical for hospital purposes.
2. Challenge spores. These are highly resistant bacterial spores (usually Bacillus stearothermophilus for steam sterilizers) impregnated on paper strips, disks, or carried in a culture media. It is assumed that when these highly resistant bacteria are killed, all other forms of microbial life are also killed. It appears that this method, involving sufficient numbers of samples and challenge spores for each load of the sterilizer, is not economically feasible for hospitals.
3. Autoclave monitors. These devices are placed at the center of all packs, or at least the largest packs. When steam penetrates to the center, a color change or melting of a pellet enclosed in a glass tube occurs. When the packs are opened for use, the user can immediately check for steam penetration. There are two general types of these monitors --a. Color change ink printed on pasteboard, and b. Pellet-in-glass-tube types -- these melt at a very exact temperature and thus give assurance that full temperature steam has penetrated to the center of packs. These monitors provide an indication of the temperature at the point in the pack where they are positioned, but cannot provide an indication of the uniformity of the temperature throughout the pack.
4. Sterilizer temperature recorder and gauges. These devices measure temperature in the sterilizer's exhaust line. While they detect many serious malfunctions of the sterilzer, they cannot measure conditions at the critical center of packs, where air pockets occur. It is, therefore, still essential that center of pack monitors be used.
5. Autoclave indicating tape. This tape is very useful to discriminate packs which have been processed through the sterilizer from those which have not. It has a time-temperature factor very similar to most paper monitors which change color and is economical to use. However, the ink on this tape and many paper monitors may change color at a temperature below that specified for normal sterilization cycles.
6. Bowie-Dick Test. The purpose of the test is to measure the uniformity of steam penetration to the center of dressing packs. It cannot measure temperature achieved at center of packs so cannot replace autoclave monitors. It consists of crossing several strips of autoclave indicating tape on a sheet of paper, then placing this at the center of a test pack. The pack is run through a normal cycle, then the tape is examined for uniformity of color change. This test has a serious limitation in its failure to distinguish between high temperatures for a short time period or lower temperatures for a longer period of time.